1. Field of Invention
The invention relates to water-in-oil bitumen dispersions. More specifically, the invention relates to water-in-oil bitumen dispersions, in which the oil phase is one or more paving asphalts and to methods for making same.
2. Description of the Prior Art
Hot mix paving compositions consist principally of aggregate and bitumen generally made by heating aggregate to temperatures exceeding 300° F. (150° C.) to quantitatively remove both surface and pore-bound moisture. Bitumen is heated to temperatures typically exceeding 290° F. to lower the viscosity of the product and make it suitable for pumping through conventional liquid transfer equipment. The heated, water-free aggregate and bitumen are then mixed in either batch or continuous mixing equipment to produce the desired paving composition. These processing steps result in the paving composition typically exceeding 290° F. upon exiting the mixing equipment. This high-temperature paving composition is typically referred to by those skilled in the art as hot-mix asphalt (“HMA”).
Elevated temperatures are needed in the production of HMA to ensure complete aggregate drying and to ensure that the bitumen-aggregate mixture has adequate fluidity to be processed easily through the hot-mix asphalt paving plant transfer equipment such as the buggy, belt, or slat conveyers used in silo storage systems. Additionally, bitumen-aggregate mixtures are produced at temperatures exceeding 290° F. to ensure that they will discharge uniformly from haul trucks, process easily through asphalt paver equipment, and compact to desired densities under compressive force of conventional, static, vibratory, or oscillatory steel and pneumatic compacting equipment.
U.S. Pat. No. 5,256,195 to Redelius (which is hereby incorporated by reference) teaches the use of anionic invert emulsions containing breaking agents in the aqueous phase of the water-in-oil emulsion in combination with slow-setting cationic emulsions as the main mixing and coating emulsion. These invert emulsions are not, however, created with bitumens suitable for construction of load-bearing pavements. Rather, they comprise chiefly low-viscosity oils, such as mineral oils and other solvents, or low-viscosity bitumens, such as bitumen flux. The use of cutter stocks in asphalt pavements results in decreases in the early compressive strength of compacted pavement. Dosages of cutter stocks, as little as 0.1% by weight of the emulsion, often decrease the compacted pavement compressive strength until such time as the cutter stock has evaporated into the atmosphere. Decreased compressive strength may result in deformation under traffic.
For ease of handling, conventional bitumen emulsions used in fast-, medium-, and slow-setting asphalt paving applications are oil-in-water emulsions exhibiting viscosities and flow characteristics suitable for pumping, mixing, and spraying. In conventional oil-in-water emulsions, the bitumen or oil phase is dispersed as stabilized droplets in a continuous phase of water. Conventional oil-in-water emulsions are, thus, dispersions of discrete bitumen droplets in water. These oil-in-water bitumen dispersions typically contain an oil phase comprising 50-75% bitumen by weight of the emulsion.
When using bitumens suitable for constructing load bearing pavements, it is not possible to exceed 75% bitumen (by weight of the finished emulsion) in the dispersed phase of a conventional oil-in-water bitumen dispersion. Unacceptable handling and transfer properties result from production of oil-in-water bitumen emulsions with over 75% bitumen (or “residue” as the non-aqueous portion of bitumen emulsions is commonly referred to by those skilled in the art). Raising temperatures to facilitate handling is not an option because the water phase of the oil-in-water emulsion would evaporate, leading to further increases in viscosity.
In contrast to oil-in-water bitumen emulsions water-in-oil bitumen dispersions consist of water dispersed in the “oil phase” of bitumen. The interface between the polar water and non-polar bitumen phases can be stabilized by the use of surface active agents, not known frequently in asphalt paving applications using conventional oil-in-water dispersions. Water-in-oil bitumen dispersions are characterized by dispersed water droplets within a continuous oil phase of bitumen. Thus, water-in-oil bitumen dispersions may be referred to as “inverted” water-in-oil emulsions, or simply “invert” emulsions for short. In water-in-oil bitumen dispersions, the dispersed water phase generally comprises less than 20% by weight of the water-in-oil emulsion.
If comprised of paving grade bitumen (either performance-graded bitumens specified by the Strategic Highway Research Program, viscosity-graded bitumens, or penetration graded bitumens), the resulting water-in-oil bitumen dispersion exhibits viscosity that prevents flow at temperatures below the boiling point of water. Poor handling and transfer properties resulting from high viscosity, thus, prevent the use of “invert” dispersions in the production of bituminous compositions for construction of load-bearing pavements using conventional hot-mix asphalt production and construction equipment. Plant engineering controls and liquid transfer equipment are not compatible with use of high-viscosity forms of bitumen liquids.
“Invert” water-in-oil bitumen dispersions made with conventional paving grade bitumens must be produced in pressure vessels to prohibit evaporation of water. Paving grade bitumens typically need to be heated to around 135° C. to fluidize sufficiently to process properly in conventional colloid mill equipment. Combination of 80 parts bitumen at 135° C. with 20 parts water leads to a finished invert emulsion exceeding 100° C. Without backpressure, boiling of the water occurs.
To overcome the limitations imposed by the viscosity of the paving grade bitumen, water-in-oil bitumen dispersions are typically produced with bitumens that have been diluted with bitumen-compatible solvents such as diesel, naphtha, gasoline, kerosene, biodiesel, waste oils, or other suitable bitumen-compatible diluent. Dilution allows reduction of the temperature of the oil phase during emulsion production (so that the water does not boil out of the finished product).
However, use of these bitumen-compatible solvents and diluents has undesirable consequences. Fugitive vapors in these solvents may pose worker health hazards or create air pollutants. Additionally, these solvents and diluents may leach into soils and groundwater supplies, causing undesired effects water and soil quality. Lastly, these solvents and diluents that remain in the bitumen in the finished, compacted pavement structure lead to a significant reduction in the stiffness of the pavement. Reduction in stiffness, in turn, leads to deformation in the pavement structure under the load of traffic. Thus, pavements containing solvents and diluents find only limited application on today's highways, being used primarily on rural, low-volume routes.
Hot mix paving compositions, based on paving grade bitumens and designed for high-traffic load-bearing asphalt pavements, are normally produced by mixing non-emulsified bitumen with aggregate at elevated temperatures (usually in excess of 150° C.). The two most common hot mix facilities, drum mix plants and batch plants, heat aggregate in a rotating kiln to extremely high temperatures to drive off all water adsorbed to the aggregate, as well as all water absorbed within the surface pores of the aggregate. As noted, quantitative removal of water is required to ensure that the finished hot mixture of aggregate and bitumen shows no moisture sensitivity in the finished pavement layer once it is transported, laid down, and compacted.
As described above, hot, dry aggregate produced in conventional hot mix operations is mixed with bitumen (which is previously liquefied by heating to temperatures far in excess of its melting point) to produce what is known in the industry as the “hot mix asphalt.” Hot mix asphalts generally must be produced, laid down, and compacted at temperatures in excess of about 150° C., as the compactability of the hot mix asphalt depends on the temperature. If the mix cools, the asphalt viscosity increases and the mixture cannot be compacted to the design density (referred to as “air voids”). When a hot asphalt-aggregate mixture produced in this way cools to temperatures below about 100° C., the handling, placement, and compaction of the mixture become extremely difficult and design densities (air voids) cannot be realized. Failure to reach design densities results in deformation or rutting of the pavement layer in the wheel paths of vehicular traffic. Failure to reach design density may yield an overly porous pavement which is susceptible to moisture intrusion and moisture-related distress.
It is a general object of the present invention to disclose a method of producing water-in-oil bituminous dispersions.
Another object of the invention relates to water-in-oil bitumen dispersions, which comprise an oil phase consisting paving asphalts of the penetration-graded, viscosity-graded, and/or penetration-graded varieties substantially free of volatile solvents.
A further object of the invention relates to water-in-oil bitumen dispersions comprising an oil phase consisting of paving asphalts of the penetration-graded, viscosity-graded, and/or penetration-graded varieties substantially free of volatile solvents, and which are made using in-line mixing operations.
A still further object of the invention relates to water-in-oil bitumen dispersions, which comprise an oil phase consisting of paving asphalt of the penetration-graded, viscosity-graded, and/or penetration-graded varieties, and which are made using in-line mixing methods involving static and/or dynamic mechanical unit operations in fixed and/or mobile asphalt mix plants of the batch, continuous, and/or dual varieties to produce asphalt paving compositions suitable for use in construction of load-bearing asphalt pavements.
Yet another object of the invention relates to water-in-oil bitumen dispersions, which comprise an oil phase of paving asphalts of the penetration-graded, viscosity-graded, and/or penetration-graded varieties, and which exhibit controllable, temperature-dependent interfacial rheology resulting from the inclusion in the dispersion of specific surfactant packages.
A still further object of the present invention is to produce water-in-oil bituminous dispersions, using surfactants, which exhibit substantially complete aggregate coating, compact to required densities in the field, and which rapidly develop load-bearing strength.
A even further object of the present invention is to produce bituminous compositions, using water-in-oil bitumen dispersions made at very high bitumen residues which are suitable for use in paving applications and which are suitable for construction of a paved road.